The present invention relates generally to optical systems and, more particularly, to an electrically tunable, optical beam steerer for use in an optical phased array antenna.
There are currently very pressing needs for rapid, large-angle pointing and scanning of laser beams of all wavelengths from the infrared through the ultraviolet. These needs include strictly military applications such as laser weapons, which require agile, high energy laser pointing, and laser radar, used for target search, acquisition, tracking and surveillance. There are also purely commercial needs such as laser light shows and laser printing, which require rapid, programmable beam steering. In addition, there are also many areas common to both military and commercial interests such as optical computing and image processing, which require rapid scanning of spatial light modulators, and optical data storage requiring rapid optical addressing. In most of these cases, the impediment to effective performance of the optical system is in the area of beam steering.
Presently available technologies are generally not sufficiently advanced to supply the need for rapid, large-angle pointing and scanning of optical beams and, in particular, of large diameter, diffraction limited carbon dioxide (CO.sub.2) laser radar beams. In many systems, optical beam steering is currently performed using rotating optical elements. Such systems typically consist of galvanometer mirrors and afocal telescopes, performance being limited to beam diameters of somewhat less than six inches, a field of view of approximately five degrees in each direction, and a frame time of approximately one second with a few thousand resolution cells and open loop random access time in the order of ten milliseconds. The capability of handling larger beams is required for higher power systems, particularly for many of the military applications for CO.sub.2 laser radar systems. Larger fields of view and larger apertures, on the order of one-half to one meter, are of great interest, and faster scan times are desired for many applications. In short, there exists a pressing need for an optical version of the versatile phased array antennas now widely used for microwave radar systems.
A static deflector for deflecting a polarized infrared beam is disclosed in U.S. Pat. No. 4,639,091, issued Jan. 27, 1987, to J.-P. Huignard et al. The Huignard et al. deflector comprises a layered square plate having as a front layer a window on which stripe electrodes are disposed. Both the window and the stripe electrodes are transparent to an incident infrared beam. A middle layer of the deflector comprises an electro-optical liquid crystal layer. The bottom layer comprises a substrate having a common electrode adjacent the liquid crystal layer. The common electrode is preferably reflective at the beam wavelength, illustratively it is a gold film; alternatively, for a deflector operating by transmission, a transparent rear plate may be used.
Huignard et al. discloses a periodic staircase waveform comprising N voltage steps which are applied to the stripe electrodes, thereby creating local variations of the refractive index in the liquid crystal layer in such a manner as to form a beam diffraction grating of adjustable period.
One of the shortcomings of the Huignard et al. deflector lies in the poor response time of the liquid crystal molecules to the application of, and, more particularly, the removal of, the control voltage. Although the molecules align themselves relatively quickly along the direction of an electric field induced by the application of a control voltage, the time required for the molecules to have their quiescent alignment restored following the removal of the control voltage may be totally unacceptable for the beam steering rates required by many applications. The Huignard et al. patent indicates that the response time may be improved by optimizing certain parameters such as the structure of the liquid crystal, its viscosity, and the operating temperature.
The present invention provides improved response time for an optical beam deflector by recognizing that the response time of liquid crystal molecules increases as the square of the thickness of the liquid crystal layer. Therefore, it is advantageous to establish as thin a layer of the liquid crystal as is possible. Nevertheless, the present invention also recognizes that for the liquid crystal layer to be effective as a beam steerer, the optical beam must travel through a sufficient thickness of liquid crystal molecules to accumulate approximately one full wave of phase shift.